(1) Field of the Invention
This invention relates to a method of manufacturing a parison extruded from an extruding head into a hollow synthetic resin product which is bent two- or three-dimensionally by using a molding device and to an apparatus for manufacturing the above product.
(2) Description of the Prior Art
FIG. 1 shows an apparatus used for a conventional method for manufacturing a hollow synthetic resin product. The apparatus comprises an extruding head 101 attached to an extruder 100 and a molding device comprising a lower molding portion 102 provided below the extruding head 101 and an upper molding portion (not shown). The hollow synthetic resin product is produced by 1) the lower molding portion 102 is horizontally moved against the extruding head 101, 2) a parison P is extruded from the head 101 and is accommodated in a groove 103 of the lower molding portion 102, 3) the upper molding portion is put on the lower molding portion 102 for molding, and then 4) a compressed air is blown into the parison P.
The above method, however, has the following problem. Since the groove 103 is generally curved as shown in FIG. 2, the molding device, which is heavy, should be moved horizontally, which requires a complicated moving mechanism and enlarges the apparatus.
The above problem can be solved by a method shown in FIG. 3 (Japanese Patent Publication 2-117810). A lower molding portion 105 is moved in an X direction. A sub-groove 106 curved in the same way as the product to manufacture and a rod 107 are used in combination to swing a curved cylinder 109 in a Y direction perpendicular to the X direction, the curved cylinder 109 being attached to an opening of an extruding head 108. In this way, the parison P is accommodated in a groove 110 of the lower molding portion 105.
However, this method also involves a problem. If the groove 110 is curved sharply, for example, almost perpendicularly, the rod 107 is stuck at a corner of a corresponding curve of the sub-groove 106.
This problem can be solved by employing another cylinder which swings in the Y direction while holding the curved cylinder 109 instead of employing the sub-groove 106 and the rod 107. This construction requires the curved cylinder 109 to be close to the lower molding portion 105 to ease the accommodation. In such construction, however, the parison P is accommodated in the state of being twisted, whereby swelling the product partially. The swelling portion is easy to crack.
The swelling of the parison P is prevented by rotating a nozzle of the extruding head 101 around a vertical axis thereof in addition to moving the lower molding portion 105 horizontally. However, friction of the nozzle and a nozzle supporting portion prevents smooth rotation of the nozzle and finally completely unables the rotation.